As a noise reduction circuit used in a television receiver or the like, those shown in FIGS. 7 and 8 are hitherto known in which a delay circuit such as a field memory (or a frame memory) is used.
FIG. 7 shows one forming an open-loop, in which input video signal S.sub.I containing noise and fed at input terminal 1 is supplied to adders 2 and 3, and is also supplied to A/D converter 4 for converting it into a digital signal. It is and then written into field memory 5, which functions as a delay circuit. The digital signal read from memory 5 is converted back into an analog signal S.sub.I1 by D/A converter 6. Accordingly, this analog signal S.sub.I1 is delayed by a field in relation to the above signal S.sub.I.
This signal S.sub.I1 is supplied to adder 2 and subtracted from the above signal S.sub.I. The difference signal S.sub.I -S.sub.I1 between signal S.sub.I and signal S.sub.I1 is obtained from adder 2 and is supplied to movement detection circuit 7 and to gain controlled amplifier 8. The above difference signal S.sub.I -S.sub.I1 is considered a noise component if there is no movement in input signal S.sub.I within a field. In this case, difference signal S.sub.I -S.sub.I1 is multiplied by coefficient k at gain controlled amplifier 8 in accordance with detection signal S.sub.M from detection circuit 7 and then subtracted from signal S.sub.I at adder 3. Therefore, output signal S.sub.o in which noise is reduced is obtained from adder 3 at output terminal 9.
When a movement is detected by movement detection circuit 7, the above difference signal S.sub.I -S.sub.I1 is considered to contain a movement component so gain controlled amplifier 8 reduces the value of K in accordance with signal S.sub.M from detection circuit 7 to reduce the quantity of the movement component subtracted at adder 3.
Movement detection circuit 7 detects the level of the above difference signal S.sub.I -S.sub.I1, and produces, detection signal S.sub.M with which coefficient K of gain controlled amplifier 8 is controlled. This movement detection circuit 7 may also be placed to compare the input signal and output signal of field memory 5 with each other as shown by dotted lines in the drawing.
FIG. 8 shows an instance forming a closed loop, in which signal S.sub.I1, that a part which is output signal S.sub.o delayed by a field in memory 5, is subtracted from signal S.sub.I at adder 2 and that difference signal S.sub.I -S.sub.I1 is passed through gain controlled 8 amplified and subtracted from signal S.sub.I at adder 3. Thus, noise contained by signal S.sub.o is gradually removed by feeding back signal S.sub.o, after delaying it by a field, so that S.sub.o is obtained from adder 3 with noise reduced.
A noise reduction circuit using a frame memory as disclosed by Patent Kokai Sho 54-157429 is known.